1. Field of the Invention
The present invention relates to a catalyst combustion apparatus which is suitably employed in a heating system or a drying system.
2. Description of Related Art
A catalyst combustion apparatus is known which is equipped with fuel supply means for supplying fuel and air supply means for supplying air, and in which a mixture of fuel supplied by the fuel supply means and air supplied by the air supply means is burned by a catalyst. In this catalyst combustion apparatus, no flame is produced; and therefore, there is no, carbon generated. Moreover, the catalyst combustion apparatus can greatly suppress the exhaust emission at the time of ignition or extinguishment, as caused in a flame combustion, so that it is remarkably effective as a heating system for an electric vehicle, which desirably has clean exhaust gas.
In order to continue the catalyst combustion in the aforementioned catalyst combustion apparatus, it is necessary to always preheat the air for combustion to activate the catalyst. If such a preheating is performed by an electric heater, there is an increase electric power consumption. Thus, it is not suitable for an electric vehicle to use such an electric heater, due to the limited battery capacity.
If the air for combustion is preheated by a burner, carbon adheres to the catalyst so that the combustion efficiency of the catalyst may deteriorate. Moreover, at ignition, the catalyst is too cold to be sufficiently activated. Accordingly, there is the problem that the exhaust emission produced by the burner may be discharged as is.
In order to solve this problem, JP-A60-30908, there provides an exhaust gas circulation pipe for connecting between the upstream side and the downstream side of the catalyst combustion apparatus, and the air for combustion is preheated by circulating a part of the exhaust gas to the upstream side of the catalyst combustion apparatus.
JP-A-4-320710, on the other hand, provides a catalyst combustion apparatus in which the raw mixed gas of fuel and air, which is supplied to a gas passage in a catalyst layer, is accelerated by passing through an injection nozzle, so that a part of the exhaust gas may be circulated by a decompressing function due to the acceleration, and the raw mixed gas is preheated.
According to these conventional combustion apparatuses, the air for combustion can be preheated to stabilize the combustion without using any separate heating means such as an electric heater or a burner.
However, in the conventional apparatus disclosed in JP-A-60-30908, the combustion apparatus must be large-sized, because the exhaust gas circulation pipe is disposed outside the combustion apparatus. Accordingly, there is a problem in that it is difficult to mount the heating system on a vehicle having limited available space.
On the other hand, in the conventional apparatus disclosed in JP-A-4-320710, since it is necessary to mix the air and the fuel in advance, an additional device for mixing is required. In addition, a plurality of gas passages are formed in the catalyst layer, and the corresponding number of injection nozzles should be provided. Thus, the number of parts is inevitably increased, the construction is complicated, and the production cost is increased.
Also, although the catalyst combustion apparatus complies with clean requirement for the exhaust gas, the catalyst combustion apparatus may have a problem concerning exhaust gas emission in case the combustion amount is smaller than a specific amount. That is, when the combustion amount is decreased so that a ratio of heat loss (mainly caused by heat transfer by convection and radiation) relative to a calorific value generated by the combustion increases, the temperature inside the combustion apparatus is decreased to suppress activation of catalytic reaction. This may result in insufficient catalytic reaction, so that the exhaust gas emission increases.